For use as recording media in disk recording-playback devices adapted to record signals on disks and reproduce signals therefrom, magneto-optical disks have been developed which permit rewriting and have a great memory capacity and high reliability. Such disks have found wide use as external memories in computers and audio visual devices.
Developed especially in recent years are techniques for achieving improved recording densities by forming lands 17 and grooves 18 alternately on the signal bearing surface of a magneto-optical disk 1 as shown in FIG. 12 and recording signals on both the land 17 and the groove 18. The lands 17 and the grooves 18 are wobbled as illustrated, and the wobbling frequency is a predetermined center frequency as frequency-modulated. A wobble signal is detected by signal reproduction, and the rotation of the magneto-optical disk is so adjusted that the wobble signal has the center frequency at all times, whereby constant linear velocity control is realized. Various items of information (wobble information) such as address information are contained in the wobble signal which is frequency-modulated as stated above. Various control operations are realized based on the wobble information at the time of signal reproduction.
With the disk recording-playback device for use with such magneto-optical disks serving as recording media, an optical head projects a laser beam onto the disk along the land or groove for signal recording or reproduction, and laser spots are formed on the signal bearing surface of the disk as shown in the form of traces in FIGS. 6(a), (b) and (c). As a result, the recording signal is recorded on the disk in crescent recording areas illustrated. Accordingly, it is desired to read the signal at the position of center of each recording area for signal reproduction. For this purpose, a reproduction synchronizing signal as shown in FIG. 6 is produced so as to read the signal as timed with the synchronizing signal.
The size of laser spots to be formed on the signal bearing surface of the magneto-optical disk is determined according to the magnitude of the laser power and the heat transfer characteristics of the disk. When the spot diameter varies as shown in FIGS. 6(a), (b) and (c), the position of the center of the crescent signal recording area correspondingly shifts as illustrated. Accordingly, even if the reproduction synchronizing signal shown in FIG. 6 has an optimum phase for the arrangement of recording areas shown in FIG. 6(b), the phase of the synchronizing signal will differ from the optimum value for the arrangement of recording areas shown in FIG. 6(a) or (c).
For example with disk recording-playback devices adapted for constant angular velocity control, the inner peripheral side of the disk and the outer peripheral side thereof are different in linear velocity to result in a corresponding difference in optimum laser power, so that the inner peripheral side of the disk and the outer peripheral side thereof differ in the optimum phase of reproduction synchronizing signal. Furthermore, the optimum phase will differ locally, for example, due to local deterioration of the disk. If the phase of the reproduction synchronizing signal is held constant in such a case, the phase will differ from the optimum value.
If the phase of the reproduction synchronizing signal differs from an optimum value P as shown in FIG. 13, the bit error rate of the reproduced signal increases, and difficulty is encountered in performing a normal reproduction operation if the bit error rate exceeds a given threshold value Rbt. No measure has been taken against an increase in bit error rate due to such a difference of the phase of the reproduction synchronizing signal.
Accordingly, a first object of the present invention is to provide a disk playback device which is capable of performing a normal reproduction operation at all times by an optimum phase of the reproduction synchronizing signal.
Further when disk recording-playback devices are in operation for reproducing or recording signals, focusing servo or tracking servo is performed for the actuator incorporated in the optical head, based on a focus error signal or tracking error signal so as to obtain a satisfactory focus state or tracking state. When the disk recording-playback device is initiated into operation, an offset adjustment is made for focusing or tracking to thereby ensure accurate focusing servo and tracking servo for the subsequent signal reproduction or recording operation.
With disk recording-playback devices, however, variations in the ambient temperature leads to the distortion of the housing or parts of the optical head, a shift of position of the optical sensor, variations in the laser wavelength, etc., altering the offset value from an optimum value and consequently impairing the accuracy of focusing servo and tracking servo. If the offset value deviates from the optimum value T greatly as shown in FIG. 14, the bit error rate of reproduced signal exceeds the threshold value Rbt, presenting difficulty in effecting normal reproduction.
Accordingly, a second object of the invention is to provide a disk playback device wherein focusing servo or tracking servo can be performed with high accuracy at all times despite variations in the ambient temperature.